The Attack knob controls how long it takes the signal above the threshold to reach full compression.
The Release knob controls how long it takes the signal, once it falls below the threshold, to return to an uncompressed state.
The Pgm (Program) button engages 3 parallel signal detectors, each tuned to operate at different speeds. This helps the compressor operate more consistently when applied to signals with a high dynamic range. If the signal stays loud for a long(ish) time, the compressor will not decay fully but will instead stay at some lesser gain-reduction, prepared to operate on the next peak.
The term ‘program’ alludes to broadcast radio; a signal which often alternated between the announcer/dj voice and music. When given such widely varied signals, a compressor needed special tuning to work correctly on both.